Temperature
9.2 Possible Future Investigations
The YBCO cuprate system has proved to be highly anisotropic in its transport properties. In order to more accurately probe the relaxation length- and time-scales along the c-axis andab-plane directions, new films and junction designs might be helpful. One method that comes to mind is to invert the layers of the films in the heterostructure, as shown in Fig. 9.1(a), which can be controlled during the film growth process. In this geometry, since the superconductor region is equipotential, the injection of quasiparticles should occur uniformly throughout the junction interface, as either a bias voltage or current is applied between the CMR and YBCOfilms. This approach effectively removes the issue of the transfer lengths of the injection current and provides a better probe ofc-axis parameters.
However, there are technical issues that involve the growth of the CMRfilm on top of the YBCO layer instead of a typical perovskite substrate, which may be an interesting materials enterprise unto itself.
A second technique involves a more difficult microfabrication technique, that is, to create an ab-plane junction, as illustrated in Fig. 9.1(b), instead of the c-axis junctions that have so far been experimented with. To create this type of junction, it is necessary to introduce lithographic patterning during the growth of the heterostructures, in order to expose a component along theab-plane in the junction. These types of devices have been widely made in low-temperature superconductors and other semiconductor applications, and has also been applied to high-Tcsystems for high-performance S-N-S junctions.
The other variation in this type of spin-injection experiment is to replace the YBCO superconductor, which so far has been the one constant in the investigation. An
Figure 9.1: Other possible geometries for future heterostructure devices.
interesting candidate for the substitution is the infinite-layern-type cuprate Sr1−xLnxCuO2 (Ln = La, Gd, Sm) superconductor, which reveals strong spectroscopic evidences for a pure s-wave pairing symmetry [134, 4]. Such substitution can provide useful new information for
several reasons. First of all, thec-axis superconducting coherence length in this material is longer than thec-axis lattice constant, thus, this system is expected to reveal more three- dimensional characteristics. It would be interesting to study the effects of spin-injection on this less anisotropic system. Second, pure s-wave pairing symmetry is fully gapped, thus, the interactions and dynamic between the injected quasiparticles with the low-energy excitations in the gapless nodal directions may no longer be an issue. It is likely that the injection of spin-polarized quasiparticles would have different effects on superconductivity in the n-type cuprates with s-wave pairing because of a fully gapped Fermi surface that requires injection of these quasiparticles at an energy larger than the superconducting gap, and these energetic spin-polarized quasiparticles would have to undergo both energy and spin relaxation processes.
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